This invention relates to a processing circuit for a magnetoresistive rotary speed sensor or the like having a comparator with switching hysteresis for converting an analog sensor signal into a digital switching signal.
Processing circuits of this kind are used in the field of rotary speed detection for converting mechanically generated rotary speed information into preferably digital, electric signals, notably in applications in an anti-lock system.
The publication "Valvo Technische Information 861105, Anwendungen der Magnetfeldsensoren KMZ 10" describes an arrangement which generates, using a gear wheel and a magnet, a magnetic alternating field which depends on the rotary speed thereof. A rotary speed sensor operating on the basis of the magnetoresistive effect comprises four resistors which depend on the magnetic field and which form a measuring branch so as to generate a resultant electric signal corresponding to the applied steady magnetic field. It is also possible to detect the rotary speed zero. The processing circuit of this circuit arrangement is shown in FIG. 33 of the cited publication and comprises a comparator which converts the bridge signal into a switching signal. Between the non-inverting input and the output of the comparator there is provided a high-ohmic resistor which produces a switching hysteresis in conjunction with the sensor bridge resistor. The offsets caused by various effects, in the sensor signal and in the comparator circuit are eliminated by the configuration of the inverting comparator input so that the potential of the non-inverting input is shifted.
Depending on the distance between the rotary speed sensor and the gearwheel, the magnetic field changes. This measuring-technical drawback opposes the aim to provide manufacturing tolerances that are as large as possible. For correct operation of the arrangement it is necessary that the offset voltages smaller than the useful signal. Because of the direct coupling of the comparator to the sensor, the properties of the sensor have an adverse effect on those of the comparator. The bridge resistors of the rotary speed sensor exhibit a temperature dependency and also a wide spread in resistance. Because the switching hysteresis also depends on these resistors, the hysteresis, unfortunately, is also temperature-dependent and spread-dependent. The same holds true for the offset correction. In the input configuration of the non-inverting input, to a first approximation a constant current is generated which flows through the bridge resistors and hence causes a voltage drop which compensates for the offset. Therefore, the offset compensation is also dependent on the temperature drift and any variations of the rotary speed sensor. Because of the unequally loaded inputs of the comparators, supply voltage fluctuations etc. cause an offset. Furthermore, the comparatively small sensor signals are applied directly to the comparator circuit and, therefore, have a substantial effect on the offset thereof. Moreover, this arrangement is too sensitive with respect to electromagnetic interference signals. These signals reach the sensor via the power lead. When the sensor is balanced, the sensor signal does not exhibit interference signals. Because the half-bridges of the sensor are unequally loaded in respect of RF interferrence radiation, the overall bridge is no longer balanced. Interference signals are conducted further in attenuated form due to the detuning.
From the product disclosure "Differenz-Hall-Sensor mit schaltendem Ausgang TLE 4920 G" published by Siemens, there is known a further arrangement which utilizes two Hall probes. Using a gearwheel, the rotary speed of a shaft is determined. The gearwheel passes the hall probes and detunes a magnetic field generated by a magnet. Consequently, the Hall probes generate different signals, the difference therebetween being a measure of the position of a tooth with respect to the probes, the frequency thereof being dependent on the rotary speed and the number of teeth of the gearwheel. Via an amplifier, this signal is applied to a high-pass filter which eliminates the offset due to temperature, ageing and mechanical construction. What is left is the desirable periodic signal which switches over a subsequent Schmitt trigger. However, vibrations cause interference signals which are suppressed by a switching hysteresis. The high-pass filter imposes a lower limit frequency so that low rotary speeds can no longer be detected and an anti-lock system would no longer be operational. Furthermore, an external capacitor is required which is mounted in the vicinity of the brake disc and which must be designed for the relevent operating circumstances. Brake-induced high temperatures notably lead to increased leakage currents and hence to offset shifts in the processing circuit.